1. Field of the Invention
The present invention relates to bandpass radomes constructed using frequency selective surfaces.
2. Description of the Related Art
Bandpass radomes built using Frequency Selective Surfaces typically use FSS elements that are approximately xcex/2 in their largest dimension at the resonant frequency of the radome. Such half-wave elements typically exhibit multiple resonances, such that at normal incidence a radome having a resonance at f0 will typically exhibit spurious resonances at 3f0, 5f0, etc. At oblique incidence, spurious resonances will also typically occur at 2f0, 4f0, etc. Moreover, such FSS radomes will also excite surface waves that travel along the surface of the radome and shed energy to produce pattern anomalies in the pattern of an antenna placed behind the radome.
The present invention solves- these and other problems by providing a bandpass radome that reduces the number of spurious resonances. Moreover, the present bandpass radome tends to suppress Transverse Magnetic (TM) and Transverse Electric (TE) surface waves over various frequency bands. In one embodiment, the bandpass radome uses high surface impedance frequency selective surfaces in a structure that is electrically thin (typically xcex/100 to xcex/50 in thickness at resonance).
In one embodiment, the radome includes a slotted FSS ground plane layer. First and second FSS patch layers are disposed above the slotted ground plane layer. Third and fourth FSS patch layers are disposed below the slotted ground plane layer. In one embodiment, each of the FSS patch layers above and below the slotted ground plane layer are electrically connected to the slotted ground plane layer by a conducting post. The conducting posts form a rodded medium. In one embodiment, the conducting posts suppress TM and TE surface waves.
In one embodiment, the FSS patch layers above and below the ground plane use square patches. In one embodiment, the square patches have rebated comers to provide clearance for the conducting posts. In one embodiment, the conducting posts are plated-through holes. In one embodiment, a dielectric layer having a first thickness separates the FSS layers above the ground plane from each other. In one embodiment, a dielectric layer having a second thickness separates the FSS layer above the ground plane and closest to the ground plane from the ground plane. In one embodiment, a dielectric layer having a third thickness separates the ground plane from the FSS layer below the ground plane that is closest to the ground plane. In one embodiment, a dielectric layer having a fourth thickness separates the two FSS layers that are below the ground plane.
In one embodiment, a plurality of capacitive FSS layers is disposed above a slotted FSS ground plane and a plurality of capacitive FSS layers is disposed below the slotted FSS ground plane. The slotted ground plane is inductive at the resonant frequency of the radome. In one embodiment, a plurality of FSS elements above the ground plane are electrically connected to the ground plane by conducting posts.